In radiography the interior of objects is reproduced by means of penetrating radiation, which is high energy radiation also known as ionizing radiation belonging to the class of X-rays, γ-rays and high-energy elementary particle rays, e.g. β-rays, electron beam or neutron radiation.
For the conversion of penetrating radiation into visible light and/or ultraviolet radiation “luminescent” substances, called “phosphors”, are used. Cathode luminescent phosphors employed e.g. in CRT screens exhibit two related luminescent characteristics: fluorescence and phosphorescence. Fluorescence is the luminescent emission of light released from the phosphor during the time of excitation by high energy radiation as from X-rays. Phosphorescence is the emission of light from the phosphor occurring after the cessation of high energy excitation. The time of phosphorescence, or rate of decay of afterglow, is denoted as persistence, usually expressed as a measurement of time required for the phosphorescence in order to reduce or decay to a ten percent level of steady-state fluorescent brightness.
In known X-ray image intensifiers for example as disclosed in U.S. Pat. No. 3,838,273, the input screen comprises a substrate such as glass or aluminum on which is deposited an X-ray sensitive radiation conversion layer, commonly referred to as a fluorescence layer or scintillator.
In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays. In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
As described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system has meanwhile been developed wherein photostimulable storage phosphors are used having, in addition to their immediate light emission (prompt emission) on X-ray irradiation, the property to store temporarily a large part of the X-ray energy. Said energy is set free by photostimulation in the form of fluorescent light different in wavelength from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photoelectronically and transformed into sequential electrical signals. A storage screen or panel coated with such phosphors is exposed to an incident pattern-wise modulated X-ray beam and as a result thereof energy is temporarily stored in the coated storage phospors, corresponding with the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are processed to produce a visible image. Stimulation light can be transformed into an electric signal by making use of a photoelectric conversion element such as e.g. a photo-multiplier. It is clear that the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography” (CR).
Recently, in the hospitals the tendency is increasing to obtain X-ray images on computer monitor immediately after X-ray exposure of the patient. By storing and transmitting that digitized information efficiency and speed of diagnosis is enhanced. Accordingly “direct radiography” (DR) providing directly digital diagnostic X-ray images, after exposure of an adapted detector panel in a radiographic apparatus, becomes preferred instead of the conventional screen/film system. The X-ray quanta are transformed into electric signals by making use of a solid-state flat detector as “image pick-up” element. Such a flat detector is commonly called a “flat panel detector” and is two-dimensionally arranged. Making use therein of a photoconductive material as a detecting means, such as a-Se, in which the negative electrical charge of an electron and the positive electrical charge of a hole are generated by the X-ray energy, said X-ray energy is directly converted into those separated electrical charges. The electrical charge thus obtained is read out as an electric signal by the read-out element, two-dimensionally arranged in a fine area unit.
Furtheron an indirect type flat panel detector is known, in which the X-ray energy is converted into light by a scintillator, and in which the converted light is converted into the electric charge by the photoelectric conversion element such as a-Si two-dimensionally arranged in a fine area unit. The electrical charge is read out again as an electric signal by the photoelectric conversion read-out element, two-dimensionally arranged in a fine area unit.
Moreover a direct radiography detector is known in which the X-ray energy is converted into light by a scintillator, and wherein the converted light is projected on one or more CCD or CMOS sensors which are arranged matrix-wise in the same plane, through a converging body such as a lens or optical fiber. In the inside of the CCD or CMOS sensor, via photoelectric conversion, and charge-voltage conversion, an electric signal is obtained in every pixel.
This type of detector is also defined, therefore, as a solid state plane detector.
The image quality that is produced by any radiographic system using phosphor screen or panel, and more particularly, within the scope of the present invention, in a digital radiographic system, largely depends upon the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be.
It is further clear that a flat phosphor screen is highly desired in order to provide a homogeneous image from a point of view of sensitivity, of noise and of image definition, also called sharpness. In view of an economically justified coating method of a phosphor or a scintillator layer onto a flexible substrate as described in EP-Applications Nos. 03 100 723, filed Mar. 20, 2003, and 04 101 138, filed Mar. 19, 2004, it is clear that said coating method within a sealed zone maintained under vacuum conditions, essentially comprising the step of vapor deposition of said phosphor or scintillator layer onto a substrate, lays burden on the choice of said flexible substrate. So in order to allow deformation at least before, during or after said step of vapor deposition a metallic sheet or web as a substrate for the vapor deposition of scintillators or phosphors is highly desired.
Not only for that reason a metallic substrate is desired, but the more for applicability (read-out) in a flat scanning apparatus in view of the stringent requirements set forth hereinbefore, as an aluminum plate shows an intrinsic tendency to curl as a function of external forces or influences. Moreover an alternative is recommended for a configuration as disclosed in EP-A 1 398 662, wherein an X-ray cassette for computed radiography is provided in a form of a hollow box comprising top and bottom, wherein said bottom side and front, rear and lateral sides thereof have a higher material stiffness than the top side and wherein said top side is a deformable carrier or support material, supporting a storage or stimulable phosphor sheet layer.
Although providing a suitable deformability, a degree of flatness as desired in applications such as computed radiography, direct radiography and classical screen/film applications mentioned before remains questionable for metallic webs or sheets, as rigidity of said plates lays burden on its suitability for use in a read-out apparatus.